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In 2012, the eruption of deep-sea volcano Havre produced an abundance of fine ash at a depth of ~ 1000 m below sea level. In this study the 2D shapes of Havre ash grains retrieved from the seafloor were compared quantitatively with those of particles generated in a suite of different fragmentation experiments, which used remelted rhyolitic rock and pumice from the eruption site. A new statistical data analysis technique, denoted as Dendrogrammatic Analysis of Particle Morphology (DAPM) is introduced. It is designed to compare large numbers of morphometric data sets containing shape information for a set of ash particles to group them by morphological similarities and to visualize these clusters in a dendrogram. Further steps involve t tests and equivalence tests and reveal morphometric differences as well as matching features. The DAPM suggests that the majority of Havre ash was thermohydraulically produced by induced fuel coolant-interaction. A subset of ash particles features an elongated tube morphology. Their morphometry matches that of particles that were experimentally produced by a combination of shearing and quenching, and we infer that the natural particles were formed by synextrusive ash-venting., This study was supported by MARSDEN grant U001616; Havre samples were obtained with NSF funding EAR1447559. T.D. is supported by the Icelandic Research Fund (Rannís) Grant Nr. 206527-051. R.J.C. was funded by Australian Research Council grants DP110102196 and DE150101190, and by US National Science Foundation grant OCE1357443. more...

Brittle magmatic fragmentation plays a crucial role in explosive eruptions. It represents the starting point of hazardous explosive events that can affect large areas surrounding erupting volcanoes. Knowing the initial energy released during this fragmentation process is fundamental for the understanding of the subsequent dynamics of the eruptive gas-particle mixture and consequently for the forecasting of the erupting column’s behavior. The specific kinetic energy (SKE) of the particles quantifies the initial velocity shortly after the fragmentation and is therefore a necessary variable to model the gas-particle conduit flow and eruptive column regime. In this paper, we present a new method for its determination based on fragmentation experiments and identification of the timings of energy release. The results obtained on compositions representative for basaltic and phonolitic melts show a direct dependence on magma material properties: poorly vesiculated basaltic melts from Stromboli show the highest SKE values ranging from 7.3 to 11.8 kJ/kg, while experiments with highly vesiculated samples from Stromboli and Vesuvius result in lower SKE values (3.1 to 3.8 kJ/kg). The described methodology presents a useful tool for quantitative estimation of the kinetic energy release of magmatic fragmentation processes, which can contribute to the improvement of hazard assessment. more...

International audience; The presence of water at volcanic vents can have dramatic effects on fragmentation and eruption dynamics, but little is known about how the presence of particulate matter in external water will further alter eruptions. Volcanic edifices are inherently "dirty" places, where particulate matter of multiple origins and grainsizes typically abounds. We present the results of experiments designed to simulate non-explosive interactions between molten basalt and various "coolants," ranging from homogeneous suspensions of 0 to 30 mass% bentonite clay in pure water, to heterogeneous and/or stratified suspensions including bentonite, sand, synthetic glass beads and/or naturally-sorted pumice. Four types of data are used to characterise the interactions: (1) visual/video observations; (2) grainsize and morphology of resulting particles; (3) heat-transfer data from a network of eight thermocouples; and (4) acoustic data from three force sensors. In homogeneous coolants with ~20% sediment, heat transfer is by forced convection and conduction, and thermal granulation is less efficient, resulting in fewer blocky particles, larger grainsizes, and weaker acoustic signals. Many particles are droplet-shaped or/and "vesicular," containing bubbles filled with coolant. Both of these particle types indicate significant hydrodynamic magma-coolant mingling, and many of them are rewelded into compound particles. The addition of coarse material to heterogeneous suspensions further slows heat transfer thus reducing thermal granulation, and variable interlocking of large particles prevents efficient hydrodynamic mingling. This results primarily in rewelded melt piles and inefficient distribution of melt and heat throughout the coolant volume. Our results indicate that even modest concentrations of sediment in water will significantly limit heat transfer during non-explosive magma-water interactions. At high concentrations, the dramatic reduction in cooling efficiency and increase in mingling help to explain globular peperite, and provide information relevant to analyses of premixing associated with highly-explosive molten fuel-coolant interactions in debris-filled volcanic vents. more...

A basic experimental study of the behavior of magma rheology was carried out on remelted volcanic rocks using wide gap viscometry. The complex composition of magmatic melts leads to complicated rheologic behavior which cannot be described with one simple model. Therefore, measurement procedures which are able to quantify non-Newtonian behavior have to be employed. Furthermore, the experimental apparatus must be able to deal with inhomogeneities of magmatic melts. We measured the viscosity of a set of materials representing a broad range of volcanic processes. For the lower viscous melts (low-silica compositions), non-Newtonian behavior is observed, whereas the high-silica melts show Newtonian behavior in the measured temperature and shear rate range (T = 1423 K − 1623 K, γ = 10 − 2 s − 1 − 20 s − 1 ). The non-Newtonian materials show power-law behavior. The measured viscosities η and power-law indexes m lie in the intervals 8 Pa s ≤ η ≤ 210 3 Pa s, 0.71 ≤ m ≤ 1.0 (Grimsvotn basalt), 0.9 Pa s ≤ η ≤ 350 Pa s, 0.61 ≤ m ≤ 0.93 (Hohenstoffeln olivine-melilitite), and 8 Pa s ≤ η ≤ 1.510 4 Pa s, 0.55 ≤ m ≤ 1.0 (Sommata basalt). Measured viscosities of the Newtonian high-silica melts lie in the range 10 4 Pa s ≤ η ≤ 310 5 Pa s. more...

Discrete explosive bursts are known from many volcanic eruptions. In maar–diatreme eruptions, they have occurred in debris-filled volcanic vents when magma interacted with groundwater, implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface. Although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material, no details are available regarding the structure of the disturbed debris between the explosion site and the surface. Field studies of diatreme deposits reveal cross-cutting, steep-sided zones of non-bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by “debris jets” driven by explosions. In order to determine the general processes and deposit geometry resulting from discrete, explosive injections of entrained particles through a particulate host, we ran a series of analogue experiments. Specific volumes of compressed (0.5–2.5 MPa) air were released in bursts that drove gas-particle dispersions through a granular host. The air expanded into and entrained coloured particles in a small crucible before moving upward into the host (white particles). Each burst drove into the host an expanding cavity containing air and coloured particles. Total duration of each run, recorded with high-speed video, was approximately 0.5–1 s. The coloured beads sedimented into the transient cavity. This same behaviour was observed even in runs where there was no breaching of the surface, and no coloured beads ejected. A steep-sided body of coloured beads was left that is similar to the cross-cutting pipes observed in deposits filling real volcanic vents, in which cavity collapse can result not only from gas escape through a granular host as in the experiments, but also through condensation of water vapour. A key conclusion from these experiments is that the geometry of cross-cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the “intrusions” that formed them. An additional conclusion is that complex structures can form quickly from discrete events. more...

In diatremes and other volcanic vents, steep bodies of volcaniclastic material having differing properties (particle size distribution, proportion of lithic fragments, etc.) from those of the surrounding vent-filling volcaniclastic material are often found. It has been proposed that cylindrical or cone-shaped bodies result from the passage of “debris jets” generated after phreatomagmatic explosions or other discrete subterranean bursts. To learn more about such phenomena, we model experimentally the injection of gas-particulate dispersions through other particles. Analogue materials (glass beads or sand) and a finite amount of compressed air are used in the laboratory. The gas is made available by rapidly opening a valve—therefore the injection of gas and coloured particles into a granular host is a brief ( more...

Molten–Fuel–Coolant Interaction (MFCI) experiments were performed using remelted foiditic rock samples from the West Eifel volcanic field (Germany). Two experimental series were carried out with one magmatic melt and two water compositions. Bi-distilled water was used in the first series (DW-1 to DW-5). In the second series (SW-1 to SW-5), the bi-distilled water was saturated (350 g L − 1 ) with sodium chloride (NaCl). For both experimental series the fragmentation history and the energy release were recorded and compared. The smallest particles (≤ 125 μm) were studied using scanning electron microscopy (SEM). Most MFCI experiments with bi-distilled water reached higher explosion intensities than the experiments with the saline water. This was accompanied by higher particle ejection velocities as well as the formation of more fine-grained and more interactive particles of angular shape. Additionally, the smallest artificial pyroclasts were examined by evolved gas analyses (EGA). The particles from the MFCI experiments with salt solutions are found to contain more sodium hydroxide (NaOH). These observations can be explained by thermodynamic arguments. In contrast to the MFCI experiments with pure water, an additional reaction occurs with saline water that results in evolution of hydrogen chloride (HCl) gas and leaves a residue of sodium hydroxide. The MFCI process with saline water consumes more enthalpy and Gibbs free energy, so that less energy is available for the explosion. With other sodium halides dissolved in the water (NaF, NaBr or NaI) the additional reaction can be predicted to have greater or lesser effects on phreatomagmatic explosions. more...

Explosive volcanic eruptions are the result of intensive magma and rock fragmentation, and they produce volcanic ash, which consists of fragments

To simulate and predict the behaviour of a lava flow, it is essential to have a thorough knowledge of its thermophysical properties. Therefore, thermal conductivity, specific heat, and viscosity of volcanic rock material were determined in a wide temperature range. Especially, the properties of the molten material were investigated in detail. The material was taken from the Pietre-Cotte lava flow located on the isle of Vulcano, north of Sicily. The thermal conductivity of the material was determined in the temperature range 293-1623 K by the hot-wire method. Melting occurs above 1100K. The specific heat was measured by differential scanning calorimetry between 347 and 1671 K. The viscosity of the lava melt was determined with a rotational viscometer HAAKE M5. The viscometer was enclosed in a high-temperature furnace optimised for the temperature range 1373-1598 K. more...

Experimental studies have been performed to evaluate pre-explosive water–melt mixes with respect to explosive volcanic molten–fuel–coolant interaction (MFCI), i.e., phreatomagmatic explosion. Remolten ultrabasic volcanic rock was used as a magma simulant. Measurement of the explosion intensity was used to determine optimal premixing conditions. A well-defined optimal range was found for the hydrodynamic mixing energy (differential flow speed of 4.2 m/s), as well as for the water/melt mass ratio (0.03 to 0.04) under experimental conditions. The mass flux of water had a minor influence on the explosion intensity. Additionally, transparent mixing experiments with silicon oil and inked water were carried out. They indicate a direct dependence of the pre-explosive water-melt interface area on the explosion intensity. The experimental results show that the contact conditions of water and melt required for explosive MFCI may easily be established in natural volcanic systems. Thus, explosive MFCI is a probable mechanism of explosive volcanism. more...

With the aim to enhance interpretation of fragmentation mechanisms during explosive volcanism from size and shape characteristics of pyroclasts experimental studies have been conducted using remelted volcanic rock (olivine-melilitite). The melt was fragmented and ejected from a crucible by the controlled release of pressurized air volumes (method 1) or by controlled generation of phreatomagmatic explosions (Molten Fuel Coolant Interaction (MFCI); method 2). Both methods were adjusted so that the ejection history of the melt was identical in both cases. The experiments demonstrate that exclusively during MFCI, angular particles in the grain size interval 32 to 130 μm are generated that show surface textures dominated by cracks and pitting. The physical process of their generation is described as a brittle process acting at cooling rates of >106 K/s, at stress rates well above 3 GPa/m2, and during ∼700 μs. In this time period the emission of intense shock waves in the megahertz range was detected, releasing kinetic energy of >1000 J. By both experimental methods, three more types of particles were produced in addition, which could be identified and related to the acceleration and ejection history of the melt: spherical particles, elongated particles, and Pele's hair. Abundance and grain size distribution of these particles were found to be proportional to the rate of acceleration and the speed of ejection but were not influenced by the experimental method used. Pele's hair occurred at ejection speeds of >75 m/s. more...

[1] The recent eruptions of Eyjafjallajokull and Grimsvotn volcanoes in Iceland demonstrate the importance of a better understanding of processes leading to the formation of volcanic ash, specifically of fine volcanic ash that poses a threat to air traffic. Continuous deformation and brittle-type experiments were carried out to better constrain these processes. The studies on short-time continuous deformation behavior of basaltic melt showed viscoelastic properties deviating from hydrodynamic Newtonian models by more than 5 orders of magnitude. High-temperature deformation experiments on basaltic rock samples revealed an increase of elastic strengths as approaching the melting regime, also pointing to a very complex behavior at the solid-ductile boundary. Understanding magma fragmentation from the “liquid” side is a challenge, but meanwhile we propose a pragmatic solution: a thermodynamic model based on fracture mechanics. This model is in agreement with experiments and observations that show that fine volcanic ash is produced by brittle-type fragmentation of magma. A critical material property was defined, characterizing the conditions for brittle fragmentation: the fracture surface energy density, which represents the critical fragmentation energy. Short-term fracture experiments using silicate glass have been performed to investigate the formation of ash-sized particles by brittle failure and to extract this critical physical property, which was found to range between 40 and 130 J/m2. This value is in good agreement to fragmentation energies determined from experiments using remelted volcanic rocks. Now there is a tool to define critical conditions for the production of volcanic ash of a specific magma type. more...

[1] Compared to “dry” atmospheric eruption of magma or “dry” magma/rock contact, intensity and time scale of heat discharge from magma to the surroundings is strongly modified by an effective coolant: water or water-sediment mixes. In the case of subaqueous or subglacial eruptions magma-water contact must take place and can result in phreatomagmatic explosions. Even if no explosions occur, rapid cooling results in the formation of pyroclasts by thermal granulation. To study this process in detail, a short-term calorimeter was built for the direct measurement of the heat-flux from a magmatic melt to a coolant. Volcanic rocks from recent eruptions in Iceland were remelted and used to produce jets of melt poured into a coolant-filled container. Particles could be produced in a non-explosive process, that are practical identical to those from natural hyaloclastites. The process' fragmentation energy is about 10% of the total heat transferred from melt to coolant. more...

The deposits from explosive volcanic eruptions (those eruptions that release mechanical energy over a short time span1) are characterized by an abundance of volcanic ash2,3. This ash is produced by fragmentation of the magma driving the eruption and by fragmenting and ejecting parts of the pre-existing crust (host rocks). Interactions between rising magma and the hydrosphere (oceans, lakes, and ground water) play an important role in explosive volcanism4,5, because of the unique thermodynamic properties of water that allow it to very effectively convert thermal into mechanical energy. Although the relative proportion of magma to host-rock fragments is well preserved in the pyroclastic rocks deposited by such eruptions, it has remained difficult to quantitatively assess the interaction of magma with liquid water from the analysis of pyroclastic deposits2,3,4,5. Here we report the results of a study of natural pyroclastic sequences combined with scaled laboratory experiments. We find that surface features of ash grains can be used to identify the dynamic contact of magma with liquid water. The abundance of such ash grains can then be related to the water/magma mass ratios during their interaction. more...

[1] Experimental studies on explosive molten fuel-coolant interaction (MFCI) using basaltic melt compositions and water as the coolant have provided insight into the physical processes of basaltic and andesitic phreatomagmatic volcanism. Abundant field evidence indicates that rhyolitic and dacitic phreatomagmatism occurs in nature, but it has not been possible until now to generate laboratory MFCI explosions from the interaction between high-silica melts and water under laboratory conditions. The high viscosity of these melts apparently prevents formation of an effective hydrodynamic premix of melt and water, the documented precursor of experimental explosive MFCI caused by mafic melts. Our new experiments utilized samples from a rhyolitic tuff ring volcano in Mexico (Tepexitl). An experimental approach was developed, in which premixing conditions were generated by mechanical deformation of the melt, leading to brittle-type fragmentation at the melt-water interface. Physical measurements recorded during laboratory explosion provide quantitative evidence for rhyolitic explosive MFCI. Additionally, a comparison of experimentally produced particles with natural ones from Tepexitl deposits show nearly identical chemical/mineralogical composition, grain size, and grain morphology. Detailed textural analysis confirmed the presence of phreatomagmatically produced particles in both experimental and natural analog particles. The results from this series of experiments indicate that under natural conditions, stress-induced magma fracturing can lead to a critical magma-water-interface growths and trigger phreatomagmatic explosions of high-silica magma. The water source for these eruptions may include shallow aquifers, surface water bodies, strong precipitation, and intrusion into ice or wet, unconsolidated sediments. more...

[1] Basaltic melt drives most of earth's volcanism. Understanding its rheology is crucial for any model of magma transport and volcanic eruption. Basaltic magma is generally treated as a quasi Newtonian liquid, but there are observations of Non-Newtonian behaviour. With a method, that allows measurement of Non-Newtonian viscosity of a representative melt (molten basaltic rock), we found a strong shear rate dependency of viscosity in a wide range of temperatures. The temperature-viscosity dependency indicates properties of the molten phase as the cause. The viscosity data are in good agreement with a power law model. more...

[1] The release of kinetic energy during explosive volcanic eruptions is a key parameter for hazard assessment and civil defense. The explosive production of volcanic ash by intensive fragmentation of magma and host rocks represents a substantial part of this energy. For cases of explosive eruption where predominantly host rock was fragmented (phreatomagmatic eruptions) to form the major part of volcanic ash, rock mechanical parameters could be measured and fragmentation energies assigned. In cases where most of the produced ash is of juvenile origin (magmatic eruptions) a general method for the determination of fragmentation energy is still lacking. In this article we introduce a thermodynamic approach that relates grain size data of the produced ash deposits to shear rates acting during the deformation of magma. With the use of a standardized fragmentation experiment the physical parameters needed to determine the specific fragmentation energy and deformation history were measured. The experiment was calibrated and tested with two case histories of the Campi Flegrei volcanic field (southern Italy). Both eruptions are classified as “most probable worst-case scenarios” during the next period of activity, to be expected within the next 10–100 years. Using the experimentally determined specific fragmentation energies, the total mass of produced ash of each eruption, and assuming an energy dissipation as observed in the experiments, the total kinetic energy release of the worst-case Campi Flegrei eruptive events to come were calculated with 25 and 40 kt TNT equivalent. more...

[1] The arrival of a new hot batch of magma changes the thermal and the chemical state of a volcanic plumbing system. Formation of hybrid magma by chemical dilution (liquid miscibility model) of different magma batches is generally accepted. Observations of drop-like domains of less evolved (newer) magma in more evolved (host) magma point to the contribution of a mechanical mixing process (liquid immiscibility). The interplay of both mechanisms seems to control the time scale from the dormant to the active state of a volcano. We carried out laboratory experiments using melts from natural volcanic rock compositions with the aim to validate model calculations for shear rate dependent domain sizes. The results confirmed the model and also the importance of hydro-mechanically mixing for chemical dilution. The key process, however, that is able to change the state of a magma reservoir on a critical time scale is hydrodynamic magma mixing. more...

[1] Thermohydraulic explosions were produced by Molten Fuel Coolant Interaction (MFCI) experiments using remelted shoshonitic rocks from Vulcano (Italy). The fragmentation history and energy release were recorded. The resulting products were recovered and analyzed with the scanning electron microscope. Fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode. These clasts relate to the thermohydraulic phase of the MFCI, where most of the mechanical energy is released; they are here called “active” particles. The total surface area of such particles is proportional to the energy of the respective explosions. Other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime. These fragments, called “passive” particles, form after the thermohydraulic phase, during the expansion phase of the MFCI. In order to verify thermohydraulic explosions in volcanic eruptions, we compared experimental products with samples from phreatomagmatic base-surge deposits of Vulcano. Ash particles from the experiments show features similar to those from the deposits, suggesting that the experiments reproduced the same fragmentation dynamics. To achieve discrimination between active and passive particles, we calculated shape parameters from image analysis. The mass of active particles in base-surge deposits was calculated. As the material properties for the natural samples are identical to the experimental ones, the energy measurements and calculations of the experiments can be applied. For a single phreatomagmatic eruption at Vulcano, a maximum mechanical energy release of 2.75 × 1013 J was calculated, representing a TNT analogue of 6.5 kt. more...

Cooling of natural silicate melts and energy transfer to the environment are controlled by the temperature-dependent thermal conductivity. We describe a method that allows a correlation between temperature-dependent strain rate and a bulk temperature during a cooling or heating process under quasisteady state conditions in a Newtonian flow regime. A rotational viscometer measured data for experimental cooling curves of remelted volcanic rock materials. From these data we can calculate the thermal conductivity of an unknown melt after we calibrate the setup with a melt of known thermal conductivity as a by-product of viscosimetry. more...